Traditionally, polymers and commodity chemicals have been prepared from petroleum-derived feedstock. As petroleum supplies have become increasingly costly and difficult to access, interest and research has increased to develop renewable or “green” alternative materials from biologically-derived sources for chemicals that will serve as commercially acceptable alternatives to conventional, petroleum-based or -derived counterparts, or for producing the same materials as produced from fossil, non-renewable sources.
One of the most abundant kinds of biologically-derived or renewable alternative feedstock for such materials is carbohydrates. Carbohydrates, however, are generally unsuited to current high temperature industrial processes. As compared to petroleum-based, hydrophobic, aliphatic, or aromatic feedstocks having as low degree of functionalization, carbohydrates such as polysaccharides are complex, over-functionalized hydrophilic materials. As a consequence, researchers have sought to produce biologically-based chemicals that can be derived from carbohydrates, but which are less highly functionalized, including more stable bi-functional compounds, such as 2,5-furandicarboxylic acid (FDCA), levulinic acid, and 1,4:3,6-dianhydrohexitols.
1,4:3,6-Dianhydrohexitols (also referred to herein as isohexides) are derived from renewable resources from cereal-based polysaccharides. Isohexides embody a class of bicyclic furanodiols that derive from the corresponding reduced sugar alcohols (D-sorbitol, D-mannitol, and D-iditol respectively). Depending on the chirality, three isomers of the isohexides exist, namely: A) isosorbide, B) isomannide, and C) isoidide, respectively the structures of which are illustrated in Scheme 1.
These molecular entities have received considerable interest and are recognized as valuable, organic chemical scaffolds for a variety of reasons. Some beneficial attributes include relative facility of their preparation and purification, the inherent economy of the parent feedstocks used in their preparation, owing not only to their renewable biomass origins, which affords great potential as surrogates for non-renewable petrochemicals, but perhaps most significantly the intrinsic chiral bi-functionalities that permit a virtually limitless expansion of derivatives to be designed and synthesized.
The isohexides are composed of two cis-fused tetrahydrofuran rings, nearly planar and V-shaped with a 120° angle between rings. The hydroxyl groups are situated at carbons 2 and 5 and positioned on either inside or outside the V-shaped molecule. They are designated, respectively, as endo or exo. Isoidide has two exo hydroxyl groups while hydroxyl groups are both endo in isomannide, and one exo and one endo hydroxyl group in isosorbide. The presence of the exo substituents increases the stability of the cycle to which it is attached. Also exo and endo groups exhibit different reactivities since they are more or less accessible depending on the steric requirements of the derivatizing reaction.
As interest in chemicals derived from natural resources is increases, potential industrial applications have generated interest in the production and use of isohexides. For instance, in the field of polymeric materials, the industrial applications have included use of these diols to synthesize or modify polycondensates. Their attractive features as monomers are linked to their rigidity, chirality, non-toxicity, and the fact that they are not derived from petroleum. For these reasons, the synthesis of high glass transition temperature polymers with good thermo-mechanical resistance and/or with special optical properties is possible. Also the innocuous character of the molecules opens the possibility of applications in packaging or medical devices. For instance, production of isosorbide at the industrial scale with a purity satisfying the requirements for polymer synthesis suggests that isosorbide can soon emerge in industrial polymer applications. (See e.g., F. Fenouillot et al., “Polymers From Renewable 1,4:3,6-Dianhydrohexitols (Isosorbide, Isommanide and Isoidide): A Review,” PROGRESS IN POLYMER SCIENCE, vol. 35, pp 578-622 (2010), or X. Feng et al., “Sugar-based Chemicals for Environmentally sustainable Applications,” CONTEMPORARY SCIENCE OF POLYMERIC MATERIALS, Am. Chem. Society, December 2010, contents of which are incorporated herein by reference.)
Another application that has received limited interest involves isohexide-derived amphiphiles, compounds that manifest discrete hydrophilic and hydrophobic zones that afford unique inter and intramolecular self-assemblies in response to environmental stimuli. Conventionally, isohexide-based amphiphilic esters are predisposed to hydrolyze, particularly in commonly employed, non-neutral aqueous matrices. An alternative domain can offer a much greater robustness to hydrolytic conditions include alkyl ethers.
To better utilize isohexides as a green feedstock, a clean and simple method of preparing the isohexides as a mono-ether that can be subsequently modified to synthesize other compounds would be welcome by those in the green or renewable chemicals industry.